I found it extremely interesting to read about how your facial expression can truly affect your mood. A smile can give you a subtle boost to your emotional state because emotions, in and of themselves, are embodied. Rosenblum explains that the act of thinking about an emotion is somewhat responsible for the subtle changes in a number of bodily states that are associated with emotion. In this, when emotions are embodied, he explains that they each correspond to a specific group of body states, both neurophysiologically and muscularly. There is a belief that the body states involve the motor actions of our faces, therefore by changing the actual facial muscular action itself, the emotion itself is then altered, albeit slightly.
My grandmother always said to us that even if your feeling down, "get up, get dressed, and get out with a smile on". She always believed that acting positive and like nothing was wrong, would in turn actually make someone feel better. Turns out, is is both accurate and inaccurate. While there are definite changes in our emotional states, as cited by the pen experiment in the book, it does not have a strong power to pull you from a severely down mood. Maybe she was able to notice a stronger correlation of that in herself. I have always felt that if I smile, i think more positively as well, and feel that this effects me significantly.